This invention relates to a method of directional radio communication and in particular, but not exclusively, to a method of signal processing for use in cellular communication networks using space division multiple access.
Cellular communication networks based on space division multiple access and the advantages associated therewith are well known. The area covered by a cellular network is divided into a plurality of cell or cell sectors. Each cell is served by a base station which transmits signals to and receives signals from mobile stations located in the cell or cell sector associated with the respective base station. In a space division multiple access system, the base transceiver station will not transmit signals intended for a given mobile station throughout the cell or cell sector but will only transmit the signal in a beam direction from which a signal from the mobile station is received.
As the beam which is transmitted by the base transceiver station may only be transmitted in a particular direction and accordingly may be relatively narrow, the transmission power is concentrated into that narrow beam. This results in a better signal to noise ratio with both the signals transmitted from the base transceiver station and the signals received by the base transceiver station. Additionally, as a result of the directionality of the base transceiver station, an improvement in the signal to interference ratio of the signal received by the base transceiver station can be achieved. The interference caused by the signal transmitted by the base station to the mobile station to other mobile stations in the same cell or adjacent cells is also reduced. This increases the capacity of the system and/or increases the quality of communication.
SDMA systems can be implemented in analogue and digital cellular networks and may be incorporated in the various existing standards such as GSM, DCS 1800, TACS, AMPS and NMT. SDMA systems can also be used in conjunction with other existing multiple access techniques based, for example, on time division multiple access (TDMA), code division multiple access (CDMA), such as that described by the US IS-95 CDMA standard and the proposed third generation standard, and frequency division multiple access (FDMA) techniques.
As is known, a signal from a mobile station will generally follow several paths to the BTS. Those plurality of paths are generally referred to as multipaths. A given signal which is transmitted by the mobile station may then be received by the base transceiver station from more than one direction due to these multipath effects.
Signals transmitted from a mobile station to a base transceiver station are known as xe2x80x9cuplinkxe2x80x9d signals and signals transmitted from a base transceiver station to a mobile station are known as xe2x80x9cdownlinkxe2x80x9d signals. The uplink communication stream received by the base transceiver station from the mobile station comprises a series of communication bursts received in successive time slots. Each received burst of the uplink communication stream includes a reference signal and a data signal and these portions in turn each comprise a succession of signal components referred to hereinafter as bits. Likewise, the downlink communication stream transmitted from the base transceiver station to the mobile station comprises a series of communication bursts transmitted in successive time slots. Each respective burst of the downlink communication stream includes a reference signal and a data signal, each of which in turn comprising a succession of signal components referred to hereinafter as bits. The reference signals of the uplink and downlink communication streams are, in this example, referred to as pilot signals to be consistent with CDMA terminology.
It has been proposed that Pilot signals transmitted from the mobile station MS be used by the receiving base station to monitor the spatial properties of the receive communication stream in order to determine optimum transmission parameters. Conventional adaptive base transceiver stations process each communication burst received in the uplink direction to determine parameters for the corresponding burst in the downlink direction. The direction of transmission to be used in the downlink communication for a given time slot is determined based on direction of arrival information estimated from the uplink communication of the corresponding time slot, the uplink and downlink signals being at different frequencies.
Circuitry within the base transceiver station determines, for each receive time slot, an angular power profile of the uplink signal impinging on the base station antenna array from the mobile station and indicates transmission parameters to be used in each transmission time slot. In practice, the determined angular power profile is supplied to signal processing and decision circuitry which executes a beam selection algorithm to determine the downlink transmission parameters. Thus, the direction of transmission for a given communication burst, including for the pilot and data signals within that burst, is determined from estimations of parameters of pilot symbols received from the mobile station during the corresponding uplink communication burst and are kept fixed for at least the duration of that burst, i.e. for the entire transmission time slot.
However, since the envelope of the signal received at the base transceiver station will depend on the combination sum of a large number of signals having phases related with their respective carrier frequencies, it can be said that the short term responses (e.g. instantaneous behaviour) of the uplink and downlink channels will be uncorrelated. That is, the uplink and downlink channels are reciprocal only in the long term. One result is that the channel and directions of signal arrival (DoA) estimated from the uplink do not correspond with those required to communicate properly with the mobile station in the downlink direction. This problem worsens in environments characterized by larger angular spreads (e.g., micro-and pico-cells) and also when the angular resolution of base station is increased (e.g., the number of antenna elements is large).
The performance of downlink is measured not only in terms of the quality of signal registered at the receiving mobile station but also taking into consideration the operative cost required to achieve that level of quality. The base station aims to achieve at the mobile station a signal quality which is sufficient to produce an acceptable and/or pre-determined quality of service with minimum expenditure of resources. Spectral efficiency has direct impact on system capacity and link performance. Improving link performance will generally require an increase in transmission power or increased use of diversity, which tend to increase the level of generated interference. The nature of interference is different from widely angular (e.g., omnidirectional/sector antennas) to narrowly angular (e.g., adaptive antennas). In the case of widely angular antennas, since the energy is evenly distributed over the whole cell/sector, the interference is characterized by a low angular density. Whereas in the case of angularly narrow antennas, the interfering energy is concentrated in the narrow beams used. In multi-rate systems proposed in wide band-CDMA standards where high-bit rate users transmit with correspondingly high power levels, the conventional use of adaptive antennas described hereinbefore will produce highly coloured spatial interference.
Embodiments of the present invention seek to provide an improved method for directional radio communication.
According to an aspect of the present invention there is provided a method of directional radio communication in a wireless communications network between a first station and a second station, said method comprising the steps of transmitting a plurality of communication bursts from said first station to said second station, each of said bursts being substantially continuous and comprising a reference signal having a plurality of reference signal components and a data signal having a plurality of data signal components wherein respective signal components of said reference and/or data signals are transmitted in substantially different directions.
Preferred methods improve link quality because they lead to improvements in spatial correlations between the uplink and downlink channels. Preferred methods also provide fast angular diversity and the efficient whitening of the generated co-channel interference. Methods embodying the invention have particular advantages in radio environments characterised by large angular spreads and/or where base transceiver stations have relatively high angular resolutions.
A number of pilot and/or data signal transmission schemes may be employed in various embodiments. In one embodiment, a number of pilot reference signal components are transmitted in different directions at different times, consecutive reference signal components being transmitted in different directions and a number of said data signal components are transmitted in different directions at different times, the order of directional transmission used corresponding to that used during transmission of said reference signal components.
In another embodiment, a number of pilot signal components are transmitted in different directions at substantially the same time and a number of said data signal components are transmitted in different directions at different times. This allows the data signal components to be transmitted without regard to the order of directional transmission used.
In another embodiment, a different spreading code is used for transmission in each direction.
In another embodiment, the transmission of pilot signals is distributed throughout the communication burst with sets of data signal components disposed therebetween.
According to another aspect of the present invention there is provided a transceiver station for directional radio communication in a wireless communications network between a first station and a second station, said transceiver station comprising means for transmitting a plurality of communication bursts from said first station to said second station, each of said bursts being substantially continuous and comprising a reference signal having a plurality of reference signal components and a data signal having a plurality of data signal components, said means being operable to transmit respective signal components of said reference signals in substantially different directions, the data signal components being transmitted in said substantially different locations.